It has been found that blow molding is a relatively inexpensive and efficient process for molding hollow plastic products having substantially uniform wall thicknesses. However, blow molded products often require a wall of increased thickness in certain areas or other type of reinforcing structures such as internal or external ribs or internal columns. In the prior art, blow molding systems are known in which the mold sections (or dies) are moved to bring two layers of parison into contact to form a double-wall segment or a reinforcing column or rib.
Referring more specifically to the double-wall process, the mold cavity typically includes a "narrow section" having a width less than twice the thickness of the wall of the parison. The motion of the dies forces two layers of parison into contact in the narrow section so that they become "press-welded" together to form a solid wall. In such known systems, the solid wall formed on the product is typically parallel to the plane of the mold cavity seam. Stated differently, the narrow section of the cavity is normally transverse to the plane of travel of the two dies.
Another prior art method of forming double-walls in a blow molding operation involves the use of collapsing or telescoping members within the dies. For example, both U.S. Pat. No. 3,050,773 issued Aug. 28, 1962 to Hagen and U.S. Pat. No. 3,843,005 issued Oct. 22, 1974 to Uhlig disclose a method in which parison is expanded into conforming contact with the die cavity, and the die cavity is then reconfigured by the movable members to create double-wall sections.
With the known prior art systems lacking collapsing mold elements, it has not been possible to form double-wall flanges which project transverse to the die cavity seams, that is, parallel to the direction of travel of the dies. On the other hand, the systems employing dies having moving internal mold elements are in general more expensive to design and manufacture, and subject to decreased longevity.